Method and apparatus which compensates for channel distortion

ABSTRACT

A method and apparatus for compensating for channel distortion is disclosed. In the present invention, equalization is performed in the treating sequence mode when the moving ghost does not exist in the channel and there is no possibility that the equalizer diverges, the data mode is cancelled and the equalization is carried out in the blind mode when the moving ghost does not exist in the channel but there is a possibility that the equalizer diverges, the equalizer is executed in the data mode when there is no possibility that the equalizer diverges but there exists the moving ghost including slowly moving ghosts in the channel, and the equalization is performed in the data mode and blind mode when the moving ghost exists and there is a possibility that the equalizer diverges.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital TV system, and moreparticularly to a method and apparatus for compensating channeldistortion in the digital TV system.

2. Discussion of Related Art

Generally, a digital TV system removes ghost signals generated in achannel using the equalizer. Specifically, in the digital TV system, thereceived signal includes a training sequence signal in a fieldsynchronous section of each field to help a receiver performequalization. The receiver compensates for distortion generated in thechannel using the training sequence. FIG. 1 shows the structure of oneframe of a conventional digital TV signal and FIG. 2 shows a structureof the field synchronizing signal of FIG. 1.

Referring to FIG. 1, a frame includes two fields. Each field has 313data segments where one including one field synchronous segmentcontaining the training sequence signal and 312 general data segments.Also, each data segment consists of a 4 symbol data segmentsynchronizing signal and 828-symbol data. FIG. 2 shows a fieldsynchronizing signal of one data segment length including a data segmentsynchronizing pattern in the first four symbols, pseudo random sequencesof PN 511, PN 63, PN 63 and PN 63 in the following symbols, andinformation related with the VSB mode is in the next symbols. Thepolarity of the second of the three PN 63 sections alternates. That is,the polarity changes from ‘1’ to ‘0’ and from ‘0’ to ‘1’. Accordingly,even and odd fields are determined according to the polarity of thesecond PN 63.

FIG. 3 shows modelling of a conventional channel having no moving ghost.When there is no moving ghost in the channel, a high definition TV(HDTV) receiver receives an original signal and ghost signal, as shownin FIG. 3. This ghost signal loaded in the channel is removed by theequalizer of the receiver using the training sequence signal in thefield synchronizing signal. When a moving ghost signal does not exist inthe channel, the level of the received signal as seen by the HDTV isalmost always uniform. Thus, the HDTV receiver is able to remove theghost using only the training sequence in each field synchronizingsection.

However, when a moving ghost is loaded on the channel, the trainingsequence is not sufficient to remove a moving ghost because the state ofthe received signal would changes constantly. For example, a movingghost generated due to an airplane, as shown in FIG. 4, varies insequence of 1→2→3. Accordingly, the conventional method of removing theghost existing in the channel using only the training sequence cannoteffectively cope with the moving ghost in the channel, resulting in adeterioration in the performance of the system.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to solve at least theproblems and disadvantages of the related art.

An object of the present invention is to provide a method and apparatusfor compensating channel distortion by detecting and removing a movingghost included in a received signal.

Another object of the present invention is to provide a method andapparatus for compensating channel distortion performing equalizationusing a data portion as well as a training sequence when a receivedsignal has a moving ghost, to thereby effectively cope with a case wherethe ghost changes quickly in a channel.

A further object of the present invention is to provide a method andapparatus which compensates for channel distortion by detecting andremoving a moving ghost that moves slowly.

A still further object of the present invention is to provide a methodand apparatus which compensates for channel distortion by performingequalization in a blind mode when it is possible for an equalizer todiverge, to thereby carry out stable equalization.

A still further object of the present invention is to provide a methodand apparatus which compensates for channel distortion by resetting achannel decoder including the equalizer to start from the beginning whenthe equalizer is already diverged, to thereby perform stableequalization.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

To achieve the objects and in accordance with the purposes of theinvention, as embodied and broadly described herein, a method forcompensating channel distortion comprises calculating a DC value frominput data, reading the DC value a predetermined number of times in aspecific cycle, detecting maximum and minimum values from the read DCvalues, judging if there is a moving ghost in the channel from thedifference between the maximum and minimum values, and judging if threeis a possibility that the equalizer diverges from the MSE of a signalequalized by the equalizer; performing the equalization in the trainingsequence mode if it is judged that the moving ghost does not exist inthe channel and if there is no possibility that the equalizer woulddiverge; carrying out the equalization in the blind mode if it is judgedthat the moving ghost does not exist in the channel but there is apossibility that the equalizer would diverge; executing the equalizationin the data mode if it is judged that the moving ghost exists in thechannel and there is no possibility that the equalizer would diverges;and performing the equalization in the data mode and blind mode when itis judged that the moving ghost exists in the channel and there is apossibility that the equalizer would diverges.

Particularly, the present invention includes judging that a moving ghostdoes not exist in the channel if the difference between the maximum andminimum values of the DC values, which are read a predetermined numberof times in a specific cycle, is smaller than a predetermined firstcritical value; cancelling the data mode when it is judged that there isno moving ghost in the channel; and deciding that there is nopossibility that the equalizer would diverge when it is judged that theMSE value of the signal equalized by the equalizer is smaller than apredetermined second critical value, to operate the equalizer in thetraining sequence mode.

The present invention also comprises deciding that there is no movingghost in the channel when it is judged that the difference between themaximum and minimum values of the DC values, read a predetermined numberof times in a specific cycle, is smaller than the predetermined firstcritical value; cancelling the data mode when it is judged that there isno moving ghost in the channel; and deciding that there is a possibilitythat the equalizer would diverge when it is judged that the MSE value ofthe signal equalizer by the equalizer is larger than the predeterminedsecond critical second critical value, to operate the equalizer in theblind mode.

The present invention further comprises deciding that the moving ghostexists in the channel when it is judged that the difference between themaximum and minimum values of the DC values, read a predetermined numberof times in a specific cycle, is larger than the predetermined firstcritical value; turning on the data mode when the moving ghost exists inthe channel and the VSB mode of the input data corresponds to a mode forterrestrial broadcasting; and deciding that there is no possibility thatthe equalizer would diverge when it is judged that the MSE value of thesignal equalized by the equalizer is smaller than the predeterminedsecond critical value, to operate the equalizer in the data mode.

The present invention further comprises deciding that the moving ghostexists in the channel when it is judged that the difference between themaximum and minimum values of the DC values, read a predetermined numberof times in a specific cycle, is large than the predetermined firstcritical value; turning on the data mode when the moving ghost exists inthe channel and the VSB mode of the input data corresponds to a mode forterrestrial broadcasting; and deciding that there is a possibility thatthe equalizer would diverge when it is judged that the MSE value of thesignal equalized by the equalizer is larger than the predeterminedsecond critical value, to operate the equalizer in the blind mode.

In another embodiment, a method for compensating channel distortionaccording to the present invention comprises calculating a DC value frominput data, reading the DC value a predetermined number of times in aspecific cycle, detecting maximum and minimum values from the read DCvalues and storing them; shifting each values of a plurality of memoriesto the next memory and storing the minimum value in the memory which isfinally left; judging if the moving ghost exists in the channel usingthe difference between the stored maximum and minimum values and thedifference between the minimum value stored in the memory and currentmaximum value, and judging if there is a possibility that the equalizerwould diverge from the MSE value of a signal equalized by the equalizer;performing the equalization in the training sequence mode when it isjudged that the moving ghost does not exist in the channel and there isno possibility that the equalizer would diverge; carrying out theequalization in the blind mode when it is judged that the moving ghostdoes not exist in the channel but there is a possibility that theequalizer would diverge; executing the equalization in the data modewhen it is judged that the moving ghost exists in the channel and thereis no possibility that the equalizer would diverge; and performing theequalization in the data mode and blind mode simultaneously when it isjudged that the moving ghost exists in the channel and there is apossibility that the equalizer would diverge.

The judgement step comprises deciding if the difference between themaximum and minimum values is smaller than a predetermined firstcritical value; sequentially comparing the minimum values stored in theplurality of memories with a currently read maximum value and judging ifthe difference between them is larger than a predetermined thirdcritical value when the difference between the maximum and minimumvalues is smaller than the predetermined first critical value; andjudging that there exists the moving ghost in the channel when thedifference between the maximum and minimum values is larger than thefirst critical value, or the difference between the value stored in eachmemory and the currently read maximum value is larger than the thirdcritical value.

In still another embodiment of the method for compensating channeldistortion according to the present invention comprises calculating a DCvalue from input data, reading the DC value a predetermined number oftimes in a specific cycle, detecting maximum and minimum values from theread DC values, calculating the average value of them and storing it;shifting each of values of a plurality of memories to the next memoryand storing the average value in the memory which is finally left;judging if the moving ghost exists in the channel using the differencebetween the stored maximum and minimum values and the difference betweenthe average value stored in the memory and current average value, andjudging if there is a possibility that the equalizer would diverge fromthe MSE value of a signal equalized by the equalizer; performing theequalization in the training sequence mode when it is judged that themoving ghost does not exist in the channel and there is no possibilitythat the equalizer would diverge; carrying out the equalization in theblind mode when it is judged that the moving ghost does not exist in thechannel but there is a possibility that the equalizer would diverge;executing the equalization in the data mode when it is judged that themoving ghost exists in the channel and there is no possibility that theequalizer would diverge; and performing the equalization in the datamode and blind mode simultaneously when it is judged that the movingghost exists in the channel and there is a possibility that theequalizer would diverge.

An apparatus which compensates for channel distortion according to thepresent invention comprises an analog/digital converter for convertingan input signal into digital data; a synchronizing signal detector fordetecting synchronizing signals from the digital data; a VSB modedetector for detecting a VSB mode from the digital data; an input MSEcalculator for calculating the MSE of the digital data; a DC calculatorfor calculating a DC value included in the digital data; an equalizerfor removing a ghost included in the digital data in at least one ofthree modes of training sequence mode, data mode and blind mode; anoutput MSE calculator for calculating the MSE of data equalized by theequalizer; and a controller for determining the equalization mode of theequalizer using the operation results of the VSB mode detector, inputMSE calculator, DC calculator and output MSE calculator, and generatinga corresponding control signal.

The controller resets the equalizer when it is judged that thesynchronizing signal detector did not detect the synchronizing signals,or the equalizer completely diverged.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 shows a VSB data frame in the related art;

FIG. 2 shows a structure of the data field synchronizing portion of FIG.1;

FIG. 3 shows modelling of a general channel having no moving ghost;

FIG. 4 shows modeling of a general channel having a moving ghost;

FIG. 5 is a block diagram of an apparatus which compensates for channeldistortion according to the present invention;

FIG. 6 is a block diagram of the equalizer of FIG. 5;

FIGS. 7A to 7D are flow charts showing a method for compensating channeldistortion according to a first embodiment of the present invention;

FIGS. 8A to 8D show different levels in the slicer of FIG. 6;

FIGS. 9A to 9C show the blind equalization of FIG. 6;

FIG. 10 shows a 8VSB signal in the time domain and process ofcalculating a signal power using it;

FIGS. 11A and 11B are flow charts showing a method for compensatingchannel distortion according to a second embodiment of the presentinvention;

FIGS. 12A and 12B are flow charts showing a method for compensatingchannel distortion according to a third embodiment of the presentinvention; and

FIGS. 13A and 13B are flow charts showing a method for compensatingchannel distortion according to a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

Generally, a predetermined DC value is inserted in a digital TV signalbefore transmission for carrier recovery at the receiver. The DC valueappears to be a pilot in the frequency spectrum, helping the receivingto recover carriers. The DC value calculated from the data input to thereceiver is fixed when a moving ghost does not exist in a channelbecause the DC value is uniform at all time. Accordingly, the presentinvention judges if the moving ghost exists in the channel using thefact that the DC value of input data is uniform when there is no movingghost in the channel, and converts the mode of an equalizer into a datamode which uses general data portions as well as the field synchronizingsignal including the training sequence for equalization when it isjudged that the moving ghost exists from the existence of ghost thatmoves very slowly. Furthermore, the present invention judges if theequalizer diverges using the mean square error (MSE) value of the inputand output of the equalizer, and converts the mode of the equalizer intoa blind mode when it is judged that there is a possibility that theequalizer diverges, adaptively operating the equalizer according tochannel state. Here, the blind mode means sequential performance of2-level slicing, 4-level slicing and 8-level slicing, having apredetermined time interval therebetween.

A described above, the present invention effectively and quickly removesthe moving ghost which varies each moment, and performs equalizationeven when there is a possibility of the divergence of equalizer, therebymaximizing the performance of the receiver in accordance with a channelstate. FIG. 5 shows an apparatus which compensates for channeldistortion according to the present invention.

Referring to FIG. 5, the apparatus includes a tuner 101 which turns adesired channel from RF signals received through an antenna and convertsthe RF signal of the tuned channel into an intermediate frequency (IF)signal; a demodulator 102 which demodulates the IF signal of the channeltuned by the tuner 101, a channel decoder 200 which removes the ghostincluded in data demodulated by the demodulator 102; a microcomputer 100which controls the removal of ghost by the channel decoder 200 and sendsa signal to reset the tuner 101 as necessary; and a reset section 103which resets the channel decoder 200 under the control of themicrocomputer 100.

The channel decoder 200 includes an analog/digital converter (ADC) 201for converting input data into 1C-bit digital data; a synchronizingsignal detector 202 for detecting synchronizing signals from the 10-bitdigital data; a VSB mode detector 203 for detecting the VSB mode fromthe 10-bit digital data; an input MSE calculator 204 for calculating theMSE of the input 10-bit digital data; a DC calculator 205 forcalculating a DC value including in the 10-bit digital data; anequalizer 206 for removing the ghost or like contained in the 10-bitdigital data; and an output MSE calculator 207 for calculating the MSEof the equalized data.

The microcomputer 100 receives the operation results of thesynchronizing signal detector 202, the VSB mode detector 203, the inputMSE calculator 204, the DC calculator 205 and the output MSE calculator207; determines the mode of the equalizer 206, being selected from oneof a training sequence mode, data mode or blind mode; and outputs acorresponding mode on/off signal to the equalizer 206.

FIG. 6 is a block diagram of the equalizer in FIG. 5. The equalizerincludes a feedforward filter 301 and a feedback filter 307. Thefeedforward filter 301 removes near-by ghost and the feedback filter 307removes long ghost and residual ghost generated by the feedforwardfilter 301. At this time, when the step size of the equalizer is large,convergence speed of the equalizer 206 becomes fast but the residualerror after convergence is severe. On the contrary, with a small stepsize, the convergence speed of the equalizer 206 becomes slow but theresidual error is minute. In the present invention, three modes of thetraining sequence mode, the data mode and the blind mode are employed asthe mode of the equalizer 206. These three modes are briefly explainedbelow.

The training sequence mode is a method of performing equalization usingthe training sequence inserted in the field synchronizing section ofeach field, as shown in FIG. 1. Since the training sequence is known,the predetermined training sequence is compared with the trainingsequence of the input data to obtain a distortion information of thechannel. The channel distortion information is then applied to thefollowing input data, to thereby compensate for the discussion on thechannel. The training sequence mode allows the receiver to carry outequalization effectively when there is no moving ghost, i.e. when thedistortion, on the channel is uniform all the time.

However, when the distortion state of the channel varies fast due to theexistence of moving ghost as shown in FIG. 4, the distortion informationof channel obtained using the first training sequence is changed beforechannel distortion information obtained using the second trainingsequence because the training sequence comes once for each field. Thus,application of the obtained channel distortion information obtainedusing the first sequence to the data following the first trainingsequence results in an incorrect compensation of data with wrong channeldistortion information.

Thus, when the channel state varies fast due to the moving ghost in thechannel, the data mode is employed. The data mode uses the data portionas well as the training sequence for equalization to quickly compensatefor the channel state being changed. That is, the data mode performsequalization using the training sequence and 8-level slicing. Moreover,the blind mode carries out equalization according to slicing rather thaninformation of the training sequence when there is a possibility of adivergence of the equalizer 206. The present invention employs threeslicing modes, 2-level, 4-level and 8-level slicing modes.

For example, when there is no moving ghost in the channel and theequalizer 206 does not diverge, the equalizer 206 is operated in thetraining sequence mode. When the moving ghost does not exist in thechannel but the equalizer 206 diverges, the equalizer 206 is operated inthe blind mode. Finally, the equalizer 206 is operated in the data modewhen it does not diverge but the moving ghost exists in the channel, andis operated in both the data mode and the blind mode when the movingghost exists in the channel and the equalizer diverges.

FIGS. 7A to 7D are flow charts showing the operation of themicrocomputer 100 for removing the moving ghost existing in the channelthrough equalization according to the first embodiment of the presentinvention. First of all, when the power is turned on, a channel changeoccurs, or the system is reset, an algorithm applied to the presentinvention is initialized (step (401). Specifically, assuming that thereis no moving ghost in the channel, the data mode and the blind mode arecancelled and the step size of the equalizer 206 is set to a minimum.Furthermore, the initial value of a divergence counter used for judgingif the equalizer 206 diverges is set to 5, and the initial values ofcounter_1 and counter_2, which are used for other purposes, are set to0. In the first embodiment of the present invention, it is assumed thatthe step size of equalizer 206 has two modes, maximum and minimum.

After the initialization, the synchronizing signal detector 202 detectsthe data segment synchronizing signal (horizontal synchronizing signal)and field synchronizing signal (vertical synchronizing signal) from theinput data, and sends the detected result to the microcomputer 100. Thatis, the synchronizing signal detector 202 detects the data segmentsynchronizing signal and the field synchronizing signal from the 10-bitdigital data. The synchronizing signal detector 202 then outputs to themicrocomputer a signal nSyncLock of ‘0’ when both the two signals aredetected and a signal nSyncLock of ‘1’ when both synchronizing signalsare not detected.

The microcomputer 100 judges if the signal NSyncLock is ‘0’ (step 402)and, when it is, proceeds to step 404 for determining the existence ofmoving ghost because ‘0’ means that the data segment synchronizingsignal and the field synchronizing signal are all detected. When thesignal nSyncLock is judged to be ‘1’, the microcomputer 100 resets thechannel decoder 200 and the tuner 101 of FIG. 5 because it means thatthe synchronizing signals are not detected (step (403). When necessarysynchronizing signals are not detected in digital data processing, thefollowing data processing is meaningless. Thus, since the synchronizingsignals may not be detected when severe noise exists or the channeldecoder 200 is incorrectly set, the channel decoder 200 and the tuner101 are reset to restart from the beginning.

A detailed explanation of the four cases will be explained below.Namely, the four cases are a case when there is no moving ghost in thechannel and the equalizer 206 does not diverge, a case when there is nomoving ghost but there is a possibility that the equalizer 206 diverges,a case when the equalizer 206 did not diverge but the moving ghostexists in the channel, and a case when the moving ghost exists and thereis a probability that the equalizer 206 diverges.

First, when there is no moving ghost and the equalizer 206 does notdiverge, the DC calculator 205 calculates the DC value loaded in the10-bit digital data and the microcomputer 100 reads this value. Here,the microcomputer 100 increases in value of the counter_1 by onewhenever it reads the calculated DC value (step 404). The DC value isstored in the inner memory of the microcomputer 100. This operation iscarried out until the value of the counter_1 reaches ‘15’ (step 405).The critical value of 15 is experimentally set and may be changedaccording to a circuit designer. If the value of the counter_1 is judgedto be ‘15,’ a maximum and minimum of the 15 DC values stored in themicrocomputer 100 are separately stored in parameters DCmax and DCmin(step 406), and the difference between them, i. e. DCmax-DCmin, iscalculated (step 407). It is judged that there is no moving ghost in thechannel if DCmax-DCmin <6. This is because the level of moving ghostwhen DCmax-DCmin <6 can be sufficiently equalized using only thetraining sequence. The critical value of 6 is also experimentally setand may be changed according to the designer.

When DCmax-DCmin <3 in step 407, it is judged that there is no movingghost, or only a minute one exists, resetting the value of the counter_2to ‘0’ (step 409). Here, the critical value of 3 is experimentally setand may be varied according to the designer. The counter_2 is a kind offlag indicating whether a moving ghost exists in the channel. A value of‘0’ means that there is no moving ghost and ‘1’ means that the movingghost exists. In the present invention, the moving ghost is judged toexist when DCmax-DCmin≧6, setting the value of the counter_2 to ‘1’(step 408). When DCmax-DCmin≧3, or <6, the value of the counter_2 ismaintained in its current state.

After varying the state of the counter_2, the current value of thecounter_2 and the VSB mode of the input data are checked (step 410). Ifthe value of the counter_2 is ‘1’ and the VSB mode of the input datacorresponds to a terrestrial 8VSB mode in step 410, the data mode is ONand the step size becomes the maximum (step 411). Otherwise, the datamode is OFF and the step size becomes the minimum (step 412). The VSBmode is checked because the moving ghost exists only in the terrestrialbroadcasting channel while the VSB mode includes 8VSB mode used forterrestrial broadcasting and 16VSB mode used for cable broadcasting.That is, when the moving ghost exists in the channel, the step size isincreased to follow the variation in the ghost state quickly because thestate of ghost changes each moment. On the contrary, the step size isdecreased when there is no moving ghost.

Subsequently, the MSE value (MSEOUT) output by the equalizer 206 ischecked in order to judge if the equalizer 206 diverges (step 413). Thatis, the microcomputer 100 judges if the MSE value (MSEOUT) calculated bythe output MSE calculator 207 of channel decoder 200 is larger than thesignal power of 8VSB (step 414). If the output MSE value of theequalizer 206 is larger than the signal power, the equalizer 206diverged completely, in which convergence is almost impossible even whenequalization is performed in the blind mode. Accordingly, when theoutput MSE value is judged to exceed the signal power in step 414, thechannel decoder 200 is rest to such that the equalizer exists thedivergence state quickly (step 415). In other words, the blind modeoperates when it is possible for the equalizer to diverge, and it ismore effective that the channel decoder 200 is reset to restart from thebeginning when the equalizer 206 diverged completely. Alternatively,only the equalizer 206 may be reset rather than resetting the channeldecoder when the equalizer 206 diverged completely. This improves theoperation speed of the system.

FIG. 10 shows the 8VSB in the time domain, where there is no distortionsuch as noise and ghost. In the ADC 201 used in the embodiment of thepresent invention, it is assumed that 168, 120, 72, 24, −24, −72, −120and −168 are assigned to the 8 levels of FIG. 10. From the assignedvalues, the signal poser of 8VSB signal in an idea case, i.e. when thereis no distortion, can be obtained according to the following Equation(1). $\begin{matrix}{\text{Signal~~power} = {\frac{(168)^{2} + (120)^{2} + (72)^{2} + (24)^{2}}{4} = 12096}} & (1)\end{matrix}$

For example, when the equalizer 206 is completely diverted, The MSEvalue after equalization becomes large than 12096. Accordingly, thepresent invention determines whether equalization is carried out in theblind mode or not when the output MSE of the equalizer 206 does notexceed the signal power. When the output MSE value (MSEOUT) is judged tobe less than the signal power in step 414, it is decided if this valueis larger than a critical value (step 416). Here, the critical valuejudges if the equalizer 206 diverges and may be experimentallydetermined by the designer.

A judgement that the equalizer 206 may be diverged is made if the outputMSE (MSEOUT) of the equalizer 206 exceeds the critical value, and thereis a high probability that the equalizer 206 converges, otherwise.Accordingly, when the output MSE value of the equalizer 206 is less thanthe critical value in step 416, the value of a divergence counter whichchecks the divergence of the equalizer 206 becomes 5 (step 424). Thevalue of the divergence counter can also be set by the designer. Then, ajudgement is made whether the data mode is OFF (step 425), and theprocess returns to step 402 to repeat the aforementioned procedure whenthe data mode is OFF in step 425.

In the second case, the moving ghost does not exist in the channel butthe equalizer 206 diverges due to noises other than the moving ghost,for example additive white Gaussian noise (AWGN). In such case, if theoutput MSE value of equalizer 206 is decided larger than the criticalvalue in step 416, the value of the divergence counter is decreased byone (step 417), and its current value is checked (step 418). The blindmode is ON when the value of the divergence counter is not ‘0’, whichmeans a great possibility of divergence. However, the blind mode iscancelled and simultaneously, the value of the divergence counterbecomes ‘5’ when the value of the divergence counter corresponds to ‘0’(step 419). That is, when it is judged that equalizer 206 diverged once,the equalizer 206 is operated five times in the blind mode to improvereliability. As a result, the equalizer 206 operates in the blind modeunless the value of the divergence counter is ‘0’ in step 418. For this,the step size of equalizer 206 becomes the maximum (Max) to allow it toconverge quickly (step 420), and then a 2-level slicing is executed(step 421).

After the 2-level slicing, a 4-level slicing is carried out (step 422),followed by a 8-level slicing step (step 423). Here, the intervalbetween the 2-level and 4-level slicings and between the 4-level and8-level slicings may be determined by the designer. Preferably, theinterval is set to approximately 24.2 ms. Referring back to FIG. 6, theslicer 312 consists of 2-level, 4-level, 8-level and 16-level slicers.As shown in FIGS. 8A to 8D, the 2, 4, 8 or 16-level slicing is executedaccording to a slicing mode. The 16-level slicer of FIG. 8D is used forcable broadcasting and not for terrestrial broadcasting.

FIGS. 9A and 9C show the blind equalization according to the presentinvention. The 2-level slicing is performed for 24.2 ms (fieldsynchronizing section) as shown in FIG. 9A, followed by the 4-levelslicing for 24.2 ms as shown in FIG. 9B, and by the 8-level slicing asshown in FIG. 9C. An error detector 315 calculates the differencebetween decision data obtained by slicing the data output by theequalizer 206 according to the slice mode selected by a select signal ofmultiplexer 314 and the output data of the equalizer. The feedforwardfilter 301 and feedback filter 307 then perform equalization employingthe difference as an error signal. Upon completion of the successive2-level, 4-level and 8-level slidings, the process returns to step 402,repeating the same loop.

The third case is when the equalizer 206 did not diverge but the movingghost exists in the channel. When the difference between the maximum(DCmax) and minimum (DCmin) of the DC value, which were read fifteentimes in step 407, is larger than ‘6’ (DCmax-DCmin≧6), it is judged thatthe moving ghost exists in the channel, setting the value of thecounter_2 to ‘1’ (step 408). When the counter_2 is set to a value of ‘1’and the mode corresponds to 8VSB (step 410), the data modes is ON andsimultaneously, the step size of equalizer 206 becomes the maximum (step411). Thereafter, the output MSE value (MSEOUT) of the equalizer 206 isread in order to check if the equalizer 206 diverges (step 413). Anexplanation when the equalizer may have diverged or completely divergedwill be omitted as it has already been described above.

That is, when the read output MSE value of the equalizer 206 is lessthan the signal power (step 414) and less than the critical value (step416), it is judged that there is a possibility that equalizer 206converges, making the value of the divergence counter ‘5’ (step 424).Then, it is checked if the data mode is OFF (step 423), and when thedata mode is ON, the input MSE and output MSE of the equalizer 206 areread and compared with each other in order to judge whether the datamode being in the on state is appropriate. Since the equalizer 206amplifies the nose when it operates in the data mode more than in thetraining sequence mode, turning on the data mode may amplify the noiserather than remove the moving ghost in the channel, affecting theequalizer 206.

Accordingly, after the data mode is ON, the input MSE (MSEIN) and outpuMSE (MSEOUT) of the equalizer 206 are read (step 426), and stored inparameters ErrIn and ErrOUT1 within the microcomputer 100, respectively(step 427). Then, the difference (ErrOUT1-ErrIN) between the two valuesis compared with ‘0’ (step 428). When the difference exceeds ‘0’, i.e.the output MSE value is larger than the input MSE value, the data modewhich was previously turned on is turned off (step 429) because theeffect or amplifying the noise is greater than the effect of removingthe ghost when the data mode is ON. If the output MSE value is notlarger than the input MSE value, the process returns to step 402 torepeat the same procedure, while maintaining the ON state of the datamode.

When the data mode is OFF in step 429, the output MSE value of theequalizer 206 is read once more to be stored in a parameter ErrOUT2 inthe microcomputer 100 in order to judge whether turning off the datamode would be appropriate (step 430), and then compared with the valuestored in parameter ErrOUT1, which was read when the data mode was ON instep 427 (step 431). When the value of ErrOUT2 is larger than the valueof ErrOUT1, i.e. when the output MSE value of the equalizer 206 afterturning off the data mode exceeds its output MSE value when the datamode was ON, the data mode is turned on again (step 423). The fact thatthe output MSE value (ErrOUT2) after turning off the data mode is largerthan the output MSE value (ErrOUT1) means that turning off the data modeimproves the effect of amplifying the noise rather than removing theghost.

If the output MSE value (ErrOUT2) of the equalizer 206 after turning offthe data mode is smaller than its output MSE value (ErrOUT1) when thedata mode was ON, the process returns to step 402 to repeat the sameprocedure, maintaining the data mode in the OFF state. This is for thepurpose of setting the mode of the equalizer 206 to allow the output MSEvalue to be as small as possible although the output MSE value of theequalizer is larger than its input MSE when both the data mode is turnedon and turned off.

The fourth case is when the moving ghost exists in the channel and theequalizer 206 diverges, and is the worst case. Thus, the data mode is ONand simultaneously, the blind mode is performed. The operation in thiscase is similar to that in the aforementioned case when the moving ghostdoes not exist but the equalizer diverges, except that the difference,DCmax-DCmin, is judged to be more than ‘6’ to turn on the data mode.That is, when the value of DCmax-DCmin corresponds to ‘6’ in step 407,this means an existence of the moving ghost in the channel. Thus, thevalue of the counter_2 is set to ‘1’ in step 408. In step 410, when thevalue of the counter_2 is ‘1’ and the VSB mode of input data correspondsto the terrestrial 8VSB, the data mode is ON and the step size becomesthe maximum (Max) (step 411).

Then, the MSE of the output of the equalizer 206 is checked to decide ifthe equalizer 206 diverges (step 413). The microcomputer 100 reads theMSE value (MSEOUT) calculated by the output MSE calculator 207 of thechannel decoder 200 and judges if it is larger than the signal power of8VSB (step 414). When the value (MSEOUT) is larger than the signalpower, the equalizer 26 is completely diverged. Thus, the channeldecoder 200 or the equalizer 206 is reset to restart the ghost removalprocedure. On the contrary, when the value (MSEOUT) does not exceed thesignal power, the equalizer 206 did not diverge yet. Thus, the value(MSEOUT) is compared with the critical value again, checking thepossibility of divergence (step 416). Here, the critical value decidesif the equalizer 206 diverges and may be experimentally determined bythe designer.

The possibility of divergence of the equalizer 206 can be judged to behigh if the output MSE value (MSEOUT) of the equalizer 206 is largerthan the critical value, and the possibility of convergence of equalizer206 can be judged to be high, otherwise. Accordingly, when the outputMSE value of the equalizer 206 is larger than the critical value in step416, the value of the divergence counter is decreased by one (step 417),and then the current value of the divergence counter is checked (step418). The blind mode is turned on when the value of the divergencecounter does not correspond to ‘0’, but it is cancelled andsimultaneously, the value of the divergence counter is set to ‘5’ whenthe value is ‘0’ (step 419).

As a result, the equalizer 206 operates in the blind mode when the valueof the divergence counter is not equal to ‘0’ in step 418. For this, thestep size of the equalizer 206 is required to become the maximum, toallow it to converge quickly (step 420), and then the 2-level slicing isperformed (step 421). After the execution of the 2-level slicing, the4-level slicing is carried out (step 422), followed by the 8-levelslicing (step 423). Upon the completion of 2, 4 and 8-level slicings,the process returns to step 402, repeating the same loop. According tothe aforementioned method, the equalizer can stably operate as fast aspossible even when it diverges as well as when the moving ghost existsin the channel.

FIGS. 11, 12 and 13 show other embodiments of the present invention,which not only effectively and quickly remove moving ghost existing inthe channel but also removes ghost which moves very slowly. In theseembodiments, the divergence counter is not employed for simplificationof the process. FIGS. 11A and 11B show the second embodiment of thepresent invention, FIGS. 12A and 12B show the third embodiment of thepresent invention, and FIGS. 13A and 13B show the fourth embodiment ofthe present invention.

Specifically, equalization is performed in the blind mode directly whenthe MSE value (MSEOUT) calculated by the output MSE calculator 207 islarger than a critical value. The equalization is carried out in thedata mode without delay when the MSE value is equal to or less than thecritical value. Referring to FIGS. 11A and 11B, when the power is turnedon, a channel conversion occurs, or the system is reset, the portion ofalgorithm to which the present invention is applied is initialized (step501). The data mode and blind mode are cancelled on the assumption thatthe moving ghost does not exist in the channel, and the step size of theequalizer 206 is set to the minimum (Min). The initial values of thecounter_1, counter_2 and counter_3, used for judging if the moving ghostexists, are set to ‘0’. Furthermore, the time is set to 20 msec, and theinitial value of memory (memory[20]−memory[1]) is set to ‘0’.

Upon the completion of initialization, the microcomputer 100 judges ifthe signal nSyncLock output from the synchronizing signal detector 202is ‘0’ (step 502). When it corresponds to ‘0’, meaning that both thedata segment synchronizing signal and field synchronizing signal aredetected, the process goes to step 504 for checking if the moving ghostexists. When the signal nSyncLock is ‘1’, meaning that the synchronizingsignals are not detected, the channel decoder 200 and the tuner 101 ofFIG. 5 are reset, to restart the process from the beginning (Step 503).Similar to the first embodiment, this operation is performed onassumption that a signal nSyncLock of ‘0’ is generated when thesynchronizing signal detector 202 detects both the data segmentsynchronizing signal and field synchronizing signal, and a signalnSyncLock of ‘1’ is generated otherwise.

When the signal nSyncLock is judged to be ‘0’ in step 502, the DCcalculator 205 calculates the DC value loaded in the 10-bit digitaldata, and the microcomputer 100 reads this value. The microcomputer 100increases the value of the counter_1 by one whenever it reads the DCvalue (step 504). The DC values read by the microcomputer are stored inthe memory inside microcomputer 100 until the value of the counter_1reaches ‘10’ (Step 505). The counter_1 indicates the number of times ofthe DC value is read by the microcomputer 100 and its value increases byone whenever the microcomputer reads the DC value. Although ‘10’ is usedas the maximum in this embodiment, the threshold value can bearbitrarily determined differently by the designer.

When the value of the counter_1 does not correspond to ‘10’, the processdirectly goes to step 517 which judges if the moving ghost exists. Thisis for the purpose of stably operating the system by checking if theequalizer 206 diverged even though it cannot be correctly judged if themoving ghost exists because the microcomputer 100 did not read the DCvalue from DC calculator 205 ten times. When the value of the counter_1is judged to be ‘10’, the value is reset to ‘0’, and the maximum value(maximum DC_Value) and the minimum value (minimum DC_Value) of the tenDC values stored in the inner memory of microcomputer 100 are separatelystored in parameters DCmax and DCmin (step 506). Thereafter, each of thetwenty memory values set in initialization step 501 shifts to the nextmemory (step 507). That is, the value of memory[19] shifts to thememory[20], the value of memory[18] shifts to the memory[19] and so on.Upon the completion of shifting the twenty memory values, the memory[1]stores the minimum value (DCmin) detected in step 508.

The values stored in the memories are used for detecting the movingghost having a small variation in the DC value with the lapse of time,i.e. slowly moving ghost. The present invention can detect even theslowly moving ghost even though it is easily judged that the slowlymoving ghost does not exist because of the small variation in the DCvalue with the lapse of time. Accordingly, the system can operateoptimally according to channel state. At this time, the value ofDCmax−DCmin is calculated to detect the fast moving ghost before thedetection of slowly moving ghost using the memories (step 509). WhenDCmax−DCmin≧6 in step 509, the moving ghost is judged to exist in thechannel, changing the value of the counter_2 to ‘1’ (step 510). Thecounter_2 is a kind of flag indicating the existence of the moving ghostin the channel, a value of ‘0’ meaning that there is no moving ghost inthe channel and a value of ‘1’ meaning that the moving ghost exists. Thereference value 6 is experimentally determined and can be changed by thedesigner.

After the value of the counter_2 is changed to ‘1’, the value of thecounter_3 is set to ‘3’ (step 511). The counter_3 makes the system judgethat the moving ghost, which had appeared in the channel, disappear whenthe value of DCmax−DCmin becomes smaller than ‘6’ three successivetimes, rather than judging that there is no moving ghost in the channelwhen the value becomes smaller than ‘6’ followed by DCmax−DCmin≧6. Eventhough the moving ghost exists in the channel, the value of DCmax−DCminmay be smaller than ‘6’. Thus, if it is judged that there is no movingghost in the channel because the value of DCmax−DCmin becomes smallerthan ‘6’ once while the moving ghost exists, to thereby cancel the datamode, the received image becomes poor. On the contrary, when the datamode is maintained in the ON state for a while after the moving ghostdisappears, the image is received without deterioration in its quality.This is the reason why the counter_3 is employed. The reference value of3 of the counter_3 may be arbitrarily varied by the designer.

Meantime, it may be judged that there is no moving ghost in the channelwhen DCmax−DCmin<6 in step 509. In this case, however, the slowly movingghost can exist in the channel. Accordingly, it is judged if there isthe moving ghost that moves very slowly in the channel before the datamode is cancelled (step 512). Specifically, the values of thememory[20]−memory[1] which stored the minimum values, DCmin, arecompared with the currently read maximum value, DCmax, whenever themicrocomputer 100 reads the DC value ten times because the differencebetween the DC values that the microcomputer 100 read ten times cannotjudge the moving ghost that moves very slowly. Here, thememory[20]−memory[1] store the minimum values of the DC values, whichare detected by the DC calculator 205 and read by the microcomputer 100ten times.

Back to step 501, the time is initialized to 20 msec. This is to allowthe microcomputer 100 to read the DC value from the DC calculator 205 incycle of 20 msec. This value can be arbitrarily determined by thedesigner. Since the minimum value of the DC values that themicrocomputer 200 read ten times by 20 msec was stored in the memory[1]after the values of the memory[20]−memory[1] shift, it can be known thatthe values stored in the memory[20]]−memory[1] are the minimum valueswhich were read twenty times in cycle of 200 msec and stored therein.Accordingly, the variation in the DC value on the channel for the periodof 200 msec×20=4000 msec (4 seconds approximately) can be obtained usingthe values stored in the memory[20]−memory[1]. Although the number ofthe memories are twenty in the embodiment of present invention, it canbe also varied by the designer.

When the DCmax value currently read and detected is sequentiallycompared with the values of the memory[20]−memory[1] previously storedtherein, and the difference between them is larger than ‘6’, it isdecided that there exists the moving ghost which changes the DC valueslowly even if DCmax−DCmin<6 in step 509. On the contrary, it is notjudged that the moving ghost does not exist in the channel until thedifference becomes smaller than ‘6’. In diff=abs(memory[i]−DCmax) ofstep 512, abs means that the absolute value is taken. The reason why theabsolute value is obtained is that the currently detected DCmax valuemay be smaller than the values stored in the memory[20]−memory[1].Accordingly, when the value of diff is larger than ‘6’ in step 512, itis decided that there is the moving ghost which moves very slowly, toset the value of the counter_2 to ‘1’ (step 510), setting the value ofthe counter_3 to 3 (step 511).

Even though the currently detected DCmax value was compared with all thevalues stored in the memory[20]−memory[1] in step 512, when thedifference between them is smaller than ‘6’, it is judged if the valueof the counter_3 corresponds to ‘0’ (step 513). This is for the purposeof keeping the data mode in the ON state, resulting from the judgementthat the moving ghost exists in the channel, for a while, instead ofcancelling it, even when the difference between the DCmax and DCmin issmaller than ‘6’ and the difference between the DCmax and the valuesstored in the memory[20]−memory[1] is also less than ‘6’.

Accordingly, when the value of the counter_3 does not correspond to ‘0’,this value is decreased by 1 and the next routine is performed (step514). When the value is ‘0’, it is decided that there is no moving ghostin the channel, resetting the counter_2 to ‘0’ (step 515). Afterresetting the counter_2 to ‘0’ in step 515, the time of 20 msec, that isset in the initialization step 501, is changed (step 516). Though thevalue of time is selected form 20 msec, 30 msec, 40 msec and 50 msec inthe present invention, it can be arbitrarily changed by the designer.Here, the value of time corresponds to the cycle by which themicrocomputer 100 reads the DC value calculated by DC calculator 205. Inother words, when it is decided that there is no moving ghost in thechannel from the judgement if the moving ghost exists using the DC valueread by 20 msec, the value of time is changed to 30 msec so thatmicrocomputer 100 reads the DC value from DC calculator 205 by 30 msec,to judge if there exists the moving ghost. Through this method, the DCvalues is read with varying the value of time up to 50 msec, to therebycheck the existence of moving ghost.

The variation in the value of the time helps detect the moving ghostwhich moves slowly in step 512. If the value of the time is 50 msec, themicrocomputer 100 reads the DC value ten times by 50 msec and repeatsthe operation if storing the minimum value of them in the memory[1].Accordingly, the values stored in the memory[20]−memory[1] correspond tothe minimum values that are read twenty times by 500 msec and storedtherein because they are the minimum values of the DC values, which areread by the microcomputer 100 ten times by 50 msec and then stored inthe memory[1] after each of the values of the memory[20]−memory[1]shifts to the next memory. As a result, it can be known the variation inthe DC value on the channel for 500 nsec×20=10000 msec (ten secondsapproximately) using the values stored in the memory[20]−memory[1]. Asdescribed above, a DC variation in the channel exists, which correspondsto four seconds approximately when the value of time is 20 msec, andthen seconds approximately when the time is 50 msec. This means that themoving ghost that moves more slowly can be detected when the time is 50msec rather than 20 msec.

Upon the execution of one of steps 505, 511, 514 and 516, the currentvalue of the counter_2 and the VSB mode of data input to the VSB modedetector 203 are checked (step 517). When the value of the counter_2 isequal to ‘1’ and the VSB mode of the data corresponds to terrestrial8VSB(=8TVSB), meaning the moving ghost exists in the channel, the datamode is turned on and the step size of the equalizer 206 becomes themaximum (Max) (step 518). That is, since the state of the moving ghostchanges every moment when it exists in the channel, the data mode isturned on to use the data portion as well as the training sequence forequalization and simultaneously, the step size of the equalizer becomesthe maximum to quickly follow the variation in the ghost.

However, when any of the two conditions in step 517 is not satisfied, itis judged if the step size is the maximum (step 519). This correspondsto a case when there was no moving ghost from the beginning or a casewhen the moving ghost existed but no longer exists in the currentchannel. When the current step size of the equalizer 206 has the maximumvalue in step 519, it means that the moving ghost existed in the channelbut no longer exists in the channel, or the equalizer 206 performed theblind mode. This is because the above two cases make the step size tothe maximum. Furthermore, when the step size is not the maximum, itmeans that there was no moving ghost from the beginning, or the channeldecoder 200 was reset. Accordingly, when it is decided that the stepsize is not the maximum in step 519, the data mode is cancelled and thestep size becomes the minimum (step 520).

When it is judged that the step size is the maximum in step 519, whichcorresponds to the case that there is no moving ghost, the data mode iscancelled and simultaneously, the step size becomes the middle value(Min) instead of the minimum (step 521). Then, the step size is changedto the minimum after a lapse of a predetermined time (step 522). Thisoperation is called a gear shift. The converging time of the equalizercan be reduced more when the step size is varied from the maximumthrough the middle value to the minimum rather than changing from themaximum to the minimum directly.

Upon the execution of one of steps 518, 520 and 522, the MSE value ofthe output by the equalizer 206 is checked in order to judge if theequalizer 206 diverges (step 523). That is, the microcomputer 100 checksof the MSE value (MSEOUT) calculated by the output MSE calculator 207 ofthe channel decoder 200 is larger than the signal power of 8VSB (step524). Here, the output MSE value of equalizer 206 becomes larger thanthe signal power when it completely diverged. In this case, theconvergence of equalizer 206 is almost impossible even if theequalization is performed in the blind mode. Accordingly, when it isjudged that the output MSE value exceeds the signal power, the channeldecoder 200 is reset such that the equalizer 206 exits out of thedivergence state quickly (step 525). In step 525, only the equalizer 206can be reset to bring about the effect similar to the case where thechannel decoder is reset, and improves the operation speed of thesystem.

In other words, the blind mode is operated when there is a possibilitythat the equalizer diverges, and the channel decoder 200 or theequalizer 206 is reset to restart the process from the beginning whenthe equalizer 206 completely diverged. Accordingly, it is determined ifthe equalization is performed in the blind mode when the output MSEvalue of the equalizer 206 is not larger than the signal power.Specifically, when the output MSE value (MSEOUT) is smaller than thesignal power in step 524, it is judged if the output MSE value is largerthan a critical value (step 526). Here, the critical value decides ifthe equalizer 206 diverges and is determined by the designer. It isjudged that there is a possibility that the equalizer 206 diverges ifthe output MSE value of the equalizer 206 is larger than the criticalvalue, and judged that there is a high possibility that it converges ifit is smaller. Accordingly, the blind mode is not executed when theoutput MSE value of the equalizer 206 is smaller than the critical valuein step 526, returning to step 502 and repeating the process.

Meantime, when the output MSE value of the equalizer 206 is larger thanthe critical value in step 526, the equalizer 206 is operated in theblind mode, allowing the system to return to its stable state quickly.For this, the step size of the equalizer 206 becomes the maximum to makethe equalizer 206 converge fast (step 527), and then the 2-level slicingis carried out (step 528). After the 2-level slicing, the 4-levelslicing is performed, followed by the 8-level slicing (Step 530). Uponthe completion of the 2, 4 and 8-level slicings, the process returns tostep 502 again, repeating the same loop.

FIGS. 12A and 12B show the third embodiment of the present invention.Only portions of FIG. 12A, which are different from FIG. 11A, will bedescribed below. Referring to FIGS. 11A and 11B, the process goes to thestep of judging if the moving ghost exists when the value of thecounter_1 which indicates how many times the microcomputer 100 reads theDC value from DC calculator 205 does not correspond to ‘10’. In FIG.12A, however, the process returns to step 604 to repeat the operation ofreading the DC value until the value of the counter_1 becomes ‘10’ whenthe microcomputer 100 did not read the DC value ten times, i.e. whenCounter_1≠10. According to an experimental result obtained by realizingthe algorithm of FIGS. 11A, 11B, 12A and 12B, the performance of thesystem by the process of FIGS. 11A and 11B is similar to that by theoperation of FIGS. 12A and 12B.

FIGS. 13A and 13B show the fourth embodiment of the present invention.Only portions of FIGS. 13A and 13B, which are different from FIGS. 11Aand 11B, will be described below. There may be various methods forstoring values in the memory[20]−memory[1]. In FIG. 11A, the minimumvalues of the DC values read by the microcomputer 100 ten times isstored in the memory[20]−memory[1]. In FIG. 13A, however, the averagevalue of the DC values read by microcomputer 100 ten times is stored inthe memories. For this, when the value of the counter_1 corresponds to‘10’ in step 705, this value is reset to ‘0’, and then the maximum value(maximum DC_Value) and minimum value (minimum DC_Value) of the ten DCvalues stored in within the memory of the microcomputer 100 areseparately stored in parameters DCmax and DCmin while the average valueof them is stored in a parameter DCvar (step 706).

Thereafter, the twenty memory value set in initialization step 701 shiftone by one (step 707). That is, the value of the memory[19] shifts tothe memory[20], the value of the memory[18] shifts to the memory[19]],and so on. After all the twenty values are moved, the memory[1] storesthe average value (DCvar) obtained in step 706 (step 708). The value ofDCmax−DCmin is calculated to detect the moving ghost (step 708).Specifically, when DCmax−DCmin≧6 in step 709, the moving ghost is judgedto exist in the channel, changing the value of the counter_2 to ‘1’ andsetting the value of the counter_3 to ‘3’ (step 711). On the other hand,when DCmax−DCmin<6 in step 709, it is judged if there is the movingghost that moves very slowly before the data mode is cancelled (step712).

In other words, the average value stored in the memory[20]−memory[1] iscompared with the average value currently read and calculated, wheneverthe microcomputer 100 reads the DC value ten times. It is decided thatthe moving ghost which slowly changes the DC value exists in the channeleven if DCmax−DCmin<6 in step 709 when the difference between thecurrent read average value (DCvar) and the values previously stored inthe memory[20]−memory[1] becomes larger than ‘6’ at least once. It isnot judged that the moving ghost does not exist in the channel until thedifference becomes smaller than ‘6’. Accordingly, when the value ofdifference of step 712 is larger than ‘6’, it is decided that there isthe moving ghost that moves very slowly, setting the value of thecounter_2 to ‘1’ and setting the value of the counter_3 to ‘3’ (step711). Meantime, when the difference between the average value (DCvar)currently obtained in step 712 and the values stored in thememory[20]−memory[1] is less than ‘6’, it is judged if the value of thecounter_3 is ‘0’ (step 713). The explanation of the following operationsare omitted because it is identical to that shown in FIGS. 11A and 11B.

As described above, according to the method and apparatus forcompensating for channel distortion of the present invention, theequalization is performed in the training sequence mode when the movingghost does not exist in the channel and there is no possibility that theequalizer diverges, and the data mode is cancelled and the equalizationis carried out in the blind mode when the moving ghost does not exist inthe channel but there is a possibility that the equalizer diverges.Furthermore, the equalizer is executed in the data mode when there is nopossibility that the equalizer diverges but there exists the movingghost including slowly moving ghost in the channel, and the equalizationis performed in the data mode and blind mode when the moving ghostexists and there is a possibility that the equalizer diverges. By doingso, the moving ghost in the channel, which changes every minute, can beeffectively and quickly removed and the equalizer is stably carried outas fast as possible even when there is a possibility of divergence.Especially, the moving ghost that moves very slowly as well as the fastmoving ghost are detected and removed, thereby improving the reliabilityof the system. Moreover, when the synchronizing signals are detectedafter initialization, there is a possibility that the equalizerdiverges, or is completely diverged. Thus, the channel decoder is restto restart the system, performing the equalization stably.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teachings canbe readily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

What is claimed is:
 1. A method for compensating for channel distortion:(a) calculating a DC value from input data, reading the DC value apredetermined number of times in a specific cycle, detecting maximum andminimum values from the read DC values, judging if there is a movingghost in the channel from a difference between the maximum and minimumvalues, and judging if there is a possibility that an equalizer divergesfrom a MSE of a signal equalized by the equalizer; (b) executing theequalization in a training sequence mode when it is judged that themoving ghost does not exist in the channel and there is no possibilitythat the equalizer diverges; (c) executing the equalization in a blindmode when it is judged that the moving ghost does not exist in thechannel but there is a possibility that the equalizer diverges; (d)executing the equalization in a data mode when it is judged that themoving ghost exists in the channel and there is no possibility that theequalizer diverges; and (e) executing the equalization in the data modeand the blind mode when it is judged that the moving ghost exists in thechannel and there is a possibility that the equalizer diverges.
 2. Amethod of claim 1, further comprising resetting a channel decoder and atuner when synchronizing signals of the input data are not detected. 3.A method of claim 1, further comprising resetting a channel decoderincluding the equalizer when it is judged that the equalizer completelydiverged.
 4. A method of claim 3, wherein the equalizer is judged to becompletely diverged when said MSE value of the signal equalized by theequalizer is larger than a signal power.
 5. A method of claim 4, whereinthe signal power is obtained by squaring values assigned to levels ofdigital data and averaging the squared values.
 6. A method of claim 1,wherein in (a): judging that there is no moving ghost in the channelwhen it is decided that the difference between said maximum and minimumvalues is smaller than a predetermined first critical value, andotherwise judging that the moving ghost exists in the channel; andjudging that there is no possibility that the equalizer diverges when itis judged that said MSE value of the signal equalized by the equalizeris smaller than a predetermined second critical value, and otherwisejudging that there is a possibility that the equalizer divergesotherwise.
 7. A method of claim 6, wherein in (a) and (b), cancellingthe data mode when it is judged that there is no moving ghost in thechannel.
 8. A method of claim 7, wherein setting a step size of theequalizer to a minimum when cancelling the data mode.
 9. A method ofclaim 1, wherein in (c) setting a step size of the equalizer to amaximum when operating the equalizer in the blind mode.
 10. A method ofclaim 1, wherein in (c) sequentially performing 2-level slicing, 4-levelslicing and 8-level slicing, having a predetermined time intervaltherebetween when operating the equalizer in the blind mode.
 11. Amethod of claim 1, wherein in (d) turning on the data mode when themoving ghost exists in the channel and when a VSB mode of the input datacorresponds to a mode for terrestrial broadcasting.
 12. A method ofclaim 11, wherein setting a step of the equalizer to a maximum whenturning on the data mode.
 13. A method of claim 11, wherein operatingthe equalizer in the data mode comprises: reading MSE values of theinput and output of the equalizer when the data mode is judged to beturned on, and comparing the two MSE values; cancelling the data mode,reading an MSE output by the equalizer when the data mode is cancelledand comparing the MSE value read when the data mode is cancelled withthe MSE value read when the data mode is ON; and turning on the datamode again when it is judged that the MSE value output by the equalizerafter the data mode is cancelled is larger than the MSE value outputwhen the data mode is ON.
 14. A method of claim 13, wherein the datamode is maintained in the ON state when it is judged that the MSE valueoutput by the equalizer is smaller than the input MSE value, andmaintained the data mode an OFF state when it is decided that the MSEvalue output when the data mode is cancelled is smaller than the MSEvalue output when the data mode is ON.
 15. A method of claim 1, whereinin (e), turning on the data mode when the moving ghost exists in thechannel and when the VSB mode of the input data corresponds to a modefor terrestrial broadcasting.
 16. A method for compensating for channeldistortion comprises: calculating a DC value from input data, readingthe DC value a predetermined number of times in a specific cycle,detecting maximum and minimum values from the read DC values and storingthe maximum and minimum values; shifting each values of a plurality ofmemories to a next memory and storing the minimum value in the memorywhich is finally left; (a) judging if a moving ghost exists in thechannel using a difference between the stored maximum and minimum valuesand a difference between the minimum value stored in the memory and acurrent maximum value, and judging if there is a possibility that anequalizer diverges from a MSE value of a signal equalized by theequalizer; (b) performing an equalization in a training sequence modewhen it is judged that the moving ghost does not exist in the channeland there is no possibility that the equalizer diverges; (c) carryingout the equalization in a blind mode when it is judged that the movingghost does not exist in the channel but there is a possibility that theequalizer diverges; (d) executing the equalization in a data mode whenit is judged that the moving ghost exists in the channel and there is nopossibility that the equalizer diverges; and (e) performing theequalization in the data mode and the blind mode when it is judged thatthe moving ghost exists in the channel and there is a possibility thatthe equalizer diverges.
 17. A method of claim 16, wherein (a) comprises:deciding if the difference between the maximum and minimum values issmaller than a predetermined first critical value; sequentiallycomparing the minimum values stored in the plurality of memories with acurrently read maximum value and judging if the difference between aminimum value and a currently read value is larger than a predeterminedsecond critical value when the difference between the maximum andminimum values is smaller than the predetermined first critical value;and judging that the moving ghost exists in the channel when thedifference between the maximum and minimum values is larger than thefirst critical value, or the difference between the value stored in eachmemory and the currently read maximum value is larger then the secondcritical value.
 18. A method of claim 16, wherein in (a), detectingwhether the moving ghost exists while changing the cycle by which the DCvalue is read.
 19. A method of claim 17, wherein, when it is judged thatthe moving ghost does not exist and a step size of the equalizer is amaximum, the data mode is cancelled and the step size is varied from amaximum through a middle value to a minimum, having a predetermined timeinterval therebetween.
 20. A method of claim 17, wherein, when it isjudged that there is no moving ghost, the data mode is cancelled aftersuccessively judging that the moving ghost does not exist apredetermined number of times.
 21. A method for compensating for channeldistortion comprising: calculating a DC value from input data, readingthe DC value a predetermined number of times in a specific cycle,detecting maximum and minimum values from the read DC values,calculating an average value of the maximum and minimum values andstoring the average value; shifting each values of a plurality ofmemories to the next memory and storing the average value in the memorywhich is finally left; (a) judging if the moving ghost exists in thechannel using a difference between the stored maximum and minimum valuesand the difference between the average value stored in the memory and acurrent average value, and judging if there is a possibility that anequalizer diverges from the MSE value of a signal equalized by theequalizer; (b) performing an equalization in a training sequence modewhen it is judged that the moving ghost does not exist in the channeland there is no possibility that the equalizer diverges; (c) carryingout the equalization in a blind mode when it is judged that the movingghost does not exist in the channel but there is a possibility that theequalizer diverges; (d) executing the equalization in a data mode whenit is judged that the moving ghost exists in the channel and there is nopossibility that the equalizer diverges; and (e) performing theequalization in the data mode and the blind mode when it is judged thatthe moving ghost exists in the channel and when there is a possibilitythat the equalizer diverges.
 22. A method of claim 21, wherein (a)comprises: deciding if the difference between the maximum and minimumvalues is smaller than a predetermined first critical value;sequentially comparing the average values stored in the plurality ofmemories with a currently read average value and judging if thedifference between a stored average value and a currently read value islarger than a predetermined second critical value when the differencebetween the maximum and minimum values is smaller than the predeterminedfirst critical value; and judging that the moving ghost exists in thechannel when the difference between the maximum and minimum values islarger than the first critical value, or the difference between thevalue stored in each memory and the currently read average value islarger than the second critical value.
 23. An apparatus whichcompensates for channel distortion, comprising: an analog/digitalconverter which converts an input signal into digital data; asynchronizing signal detector which detects synchronizing signals fromthe digital data; a VSB mode detector which detects a VSB mode from thedigital data; an input MSE calculator which calculates a MSE of thedigital data; a DC calculator which calculates a DC value included inthe digital data; an equalizer which removes a ghost included in thedigital data in at least one of a plurality of modes; an output MSEcalculator which calculates the MSE of data equalized by the equalizer;and a controller which determines the equalization mode of the equalizerusing the operation results of the VSB mode detector, the input MSEcalculator, the DC calculator and the output MSE calculator, andgenerates a corresponding control signal.
 24. An apparatus of claim 23,wherein the controller resets the equalizer when it is judged that thesynchronizing signal detector did not detect the synchronizing signals,or the equalizer completely diverged.
 25. An apparatus of claim 23,wherein the modes of the equalizer is a training sequence mode, a datamode and a blind mode.